Fallen angels and angels were created as a static number. The only increase noted in the past was when they were having children.
They would have been the makers of all the ancient monuments we have and the destruction would have been part of the flood. The universe should have many such cities that will be abandoned when mankind gets there as all angelic beings will have moved to a new perfected heaven when they can can be given in marriage.
Adam was the first created man. The other beings in Ge:1 were modeled after these beasts and any remains of man is the remains of primates, beasts of the field with the faces of men.

Re:4:7:
And the first beast was like a lion,
and the second beast like a calf,
and the third beast had a face as a man,
and the fourth beast was like a flying eagle.